Dinitrogen (N₂) fixers (diazotrophs) drive primary productivity by supplying reactive nitrogen to marine ecosystems and promoting CO₂ uptake. N₂ fixation is considerably well-known and explored in the low latitudes of the Atlantic and Pacific Oceans. The Indian Ocean, however, remains a widely under-sampled region and an overlooked contributor to the global nitrogen budget. Here, we investigated N₂ fixation activity, diazotroph community composition, and diazotroph abundance across the multi-frontal system that separates the oligotrophic waters of the south Indian Ocean gyre from the HNLC waters of the Southern Ocean in the context of the nutrient and trace metal environment. We find a sharp contrast in the distribution of diazotroph groups across the frontal system, with cyanobacterial diazotrophs preferring warm oligotrophic waters in the gyre and non-cyanobacterial diazotrophs being more abundant in the cold nutrient-rich waters of the southern ocean. Our results show a well-defined succession of diazotroph communities associated with the southern Indian Ocean fronts. These findings expand the range of biomes in which diazotrophy can be observed, and provide new insights into diazotrophy in an understudied region.

Primary Presenter: Subhadeep Chowdhury, Mediterranean Institute of Oceanography, Aix Marseille University (subhadeep.chowdhury@mio.osupytheas.fr)

Authors:

Hugo Berthelot,   ()

Christian Furbo Reeder,   ()

Corentin BAUDET,   ()

David González Santana,   ()

Stéphane L'Helguen,   ()

Jean-François Maguer,   ()

Carolin Löscher,   ()

Arvind Singh,   ()

Stéphane Blain,   ()

Nicolas Cassar,   ()

Sophie Bonnet,   ()

Hélène Planquette,   ()

Mar Benavides,   ()